Takushi Iimori

Dirac Fermions in Borophene

Honeycomb structures of group IV elements can host massless Dirac fermions with nontrivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the β_{12} sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the β_{12} sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

Direct evidence of metallic bands in a monolayer boron sheet

The search for metallic boron allotropes has attracted great attention in the past decades and recent theoretical works predict the existence of metallicity in monolayer boron. Here, we synthesize the \b{eta}12-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boron-derived bands using angle-resolved photoemission spectroscopy. The Fermi surface is composed of one electron pocket at the S point and a pair of hole pockets near the X point, which is supported by the first-principles calculations. The metallic boron allotrope in \b{eta}12 sheet opens the way to novel physics and chemistry in material science.

Initial stage of Ag growth on Ge(001) surfaces at room temperature

Abstract Using scanning tunneling microscopy, we studied Ge(001) surfaces covered with an average of approximately 0.8 monolayers of silver. In the deposition at room temperature, we observed that Ag predominantly grew in a three-dimensional (3D) mode on bare Ge substrates, corresponding to Volmer–Weber growth. At the same time, we found two-dimensional (2D) Ag islands elongated in the dimer-row direction with one monolayer height, though the density of the 2D islands was smaller than one-fifth of that of the 3D islands. Images with atomic resolution showed stripe patterns on the 2D islands and enhancement of asymmetry of the Ge dimers on both sides of the islands at high positive bias voltages.

An Atomic Seesaw Switch Formed by Tilted Asymmetric Sn-Ge Dimers on a Ge (001) Surface

When tin (Sn) atoms are deposited on a clean germanium (Ge) (001) surface at room temperature, buckled dimers originating from the Sn atoms are formed at the Ge-dimer position. We identified the dimer as a heterogeneous Sn-Ge dimer by reversing its buckling orientation with a scanning tunneling microscope (STM) at 80 kelvin. An atomic seesaw switch was formed for one-dimensional electronic conduction in the Ge dimer–row direction by using the STM to reversibly flip the buckling orientation of the Sn-Ge dimer and to set up standing-wave states.

Proving Nontrivial Topology of Pure Bismuth by Quantum Confinement

The topology of pure Bi is controversial because of its very small (∼10  meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14-202 bilayer Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to ∼10  meV. Detailed analyses on the phase shift of the confined wave functions precisely determine the surface and bulk electronic structures, which unambiguously show nontrivial topology. The present results not only prove the fundamental property of Bi but also introduce a capability of the quantum-confinement approach.

Observing hot carrier distribution in an n-type epitaxial graphene on a SiC substrate

Hot carrier dynamics in the Dirac band of n-type epitaxial graphene on a SiC substrate were traced in real time using femtosecond-time-resolved photoemission spectroscopy. The spectral evolution directly reflects the energetically linear density of states superimposed with a Fermi–Dirac distribution. The relaxation time is governed by the internal energy dissipation of electron–electron scattering, and the observed electronic temperature indicates cascade carrier multiplication.

Graphene nanoribbons on vicinal SiC surfaces by molecular beam epitaxy

We present a new method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consist of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide of (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1-100], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high density GNRs, enable us to observe the electronic structure at K-points by angle-resolved photoemission spectroscopy, showing clear band-gap opening of at least 0.14 eV.

Laser-induced desorption from silicon (111) surfaces with adsorbed chlorine atoms

We have studied the initial stage of the laser-induced reaction of silicon surfaces with adsorbed chlorine atoms in ultrahigh vacuum, by measuring the species desorbing from the surfaces. In particular, our studies have focused on photo-chemical etching without laser-induced thermal heating. We found that the primary species desorbing from Cl-saturated Si(111) surfaces is the molecule and that the desorption efficiency with 2.3, 3.5 and 4.7 eV photons is significantly enhanced with respect to that for 1.2 eV photons. The results of previous STM studies are discussed and a possible mechanism for the photo-chemical etching is proposed.

Effects of Pb Intercalation on the Structural and Electronic Properties of Epitaxial Graphene on SiC

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

Invasive growth of Co on (2×22)R45° reconstructed O∕Cu(001)

Submonolayer growth of Co on the reconstructed Cu(001)(2×22)R45°–O surface has been investigated by scanning tunneling microscopy. Cu atoms are displaced from the Cu(001)(2×22)R45°–O structure by incoming Co atoms and subsequently aggregate into elongated islands. The deposited Co atoms are randomly distributed in the oxygen adsorbed surface as individual atoms and clusters at low coverages [⩽0.4 monolayers (ML)]. For larger coverages (⩾0.5 ML), compact fcc Co patches are formed. The adsorbed oxygen acts as a surfactant. Interfacial intermixing is reduced when Co is deposited on the Cu(001)(2×22)R45°–O surface.